That said, it's one thing not to endorse a company based on competitive reasons. It's another to do so based on ethical concerns- smoking/vaping is a pretty justifiable reason.
I think the debate is not so much "yay, vaping!" as, "why vaping and not tobacco or alcohol?" (somebody helpfully posted some examples earlier) - which would actually be easier to justify (there's a genuine concern about the dangers of vaping, but the dangers of alcohol and tobacco are far better proven - the latter being a large part of some peoples' case
for vaping). That inconsistency makes people suspect that the "ethical" reasons are being used as am umbrella for something rather more self-interested.
As I've said in another post, my guess is that the lawyers may have spotted an argument that would make the iPhone part of the "paraphernalia" of vaping - or some other specific aspects of vaping apps that risks making Apple partially liable (but that's just speculation). If so, that would be a perfectly valid reason - it just wouldn't sound quite so virtuous.
When massive retailers decide to stop selling tobacco products no one accused them of anti-trust violations.
True. However, a bit of googling will soon show up plenty of other concerns about the power of large retailers to distort the market. Also: Walmart don't sell wFridges that only accept groceries bought at Walmart (we all know that Amazon will beat them to that) - the digital world offers degrees of anti-competitiveness unprecedented in meatspace.
To re-iterate: if Apple have banned vaping apps, as far as I'm concerned, nothing of value has been lost. The issue is whether Apple should be
able to choose what you can do with your iPhone (esp. when the only realistc market choice is Apple vs. Google).